查询了相关资料后，并做了基本测试，确认.map(_._2)的含义如下
map(_._n)表示任意元组tuple对象,后面的数字n表示取第几个数.(n>=1的整数)
val p=List((“hello”,35,1.50),(“nihao”,36,1.78))
res9: List[(String, Int, Double)] = List((hello,35,1.5), (nihao,36,1.78))
scala> p.map(_._1)
res10: List[String] = List(hello, nihao)
scala> p.map(_._2)
res11: List[Int] = List(35, 36)
scala> p.map(_._3)
res12: List[Double] = List(1.5, 1.78)
原文链接：https://blog.csdn.net/happyfreeangel/article/details/84324716
